Such electric motor reduced in the cogging torque is disclosed, for example, Unexamined Japanese Patent Publication No. H4-105530 (document 1) or Unexamined Japanese Patent Publication No. 2000-209795 (document 2).
The electric motor disclosed in document 1 has its stator core divided into a tooth section and a yoke section, and the concentrated wound tooth section is engaged with the yoke section. In this configuration, the coil space factor in the slot is 60% or more. Further, the gap between tips of the tooth section, that is, the slot open size is 0.5 mm or less, and hence the Carter coefficient is larger, and the change of magnetic resistance in relation to the rotor is smaller. It is therefore disclosed that the magnetic loss of the stator can be decreased, while the cogging torque can be reduced to half.
FIG. 10 is a diagram showing the relation of slot open angle/pole angle and induced voltage and cogging torque, in a conventional electric motor as disclosed in document 1. In FIG. 10, the induced voltage (Back EMF, Back Electromotive Force) is indicated by a solid line, and the cogging torque is indicated by a dotted line. The ratio of slot open angle/pole angle shows the rate of the slot open angle to the pole angle.
It was hitherto believed, as shown in FIG. 10, that the cogging torque could be reduced by setting the slot open angle smaller provided the pole angle be the same. Document 1 discloses a technology of reducing the cogging torque by setting the slot open size to 0.5 mm or less.
The electric motor disclosed in document 2 is composed to link the outer circumferences of stators by a notched portion which is a part of laminating thin steel plates. In such configuration, the space factor is increased, and a high output, and a small and light structure may be realized. At the same time, the cogging torque can be suppressed.
However, in the technologies of document 1 and document 2, since the slot open size is smaller, a leakage flux may occur between adjacent teeth. As a result, the output torque may be reduced.